Dose detectors for radiotherapy are normally used for a pre-treatment verification of the radiation dose distribution planned for certain treatment procedures. Dosimetric detectors are known to comprise matrixes of radiation-sensible elements by which it is possible to draw instantly a response in digital form to be compared with the expectations of the system for the planning of radiotherapeutic treatment.
The above said dosimetric detectors exhibit some drawbacks.
A first drawback is given by the poor spatial resolution due to the dimension of the sensible elements and to the granularity.
Generally, the main cause of the poor spatial resolutions is the predetermined value of the minimum pitch (centre-to-centre distance) between the sensible elements, which is in the order of 7 mm.
A second drawback is given by the fact that, the sensible elements used in radiotherapy, in general, are produced from standard crystalline silicon (e.g. Czochralski) and, for this reason, they are damaged by lattice defects induced by the passage of the same radiation.
The macroscopic damage due to the rise of lattice-disorder is revealed both by a reduced sensibility, caused by the trapping of minority charge-carriers generated by radiation in the lattice defects, and by an increment of the leakage current due to the production of defects which act as generation-recombination centres.
Under these conditions, the sensibility of the device is directly proportional to the diffusion length of the minority carriers L.
Generally, L is in the order of 100-400 μm, while the total thickness S of the device is in the order of 300-500 μm; under these operating conditions, the active thickness W is L and not S.
Having demonstrated that the sensibility is a function of the absorbed dose, with a very strong dependence up to 5 kGy and that, after this value, the reduction becomes less important and its trend linear, the known detectors, in order to stabilize the sensibility, provide for a pre-radiation up to doses of 10 kGy with 20 MeV-electrons, so as to operate, during the radiotherapy, within the region in which the decay of sensibility is less pronounced.
After the pre-radiation, in fact, the dosimetric detector is used in clinical radiotherapeutic applications with radiation doses which are actually less intense but which, however, are cause for a slight reduction of sensibility, thereby calling for frequent and complex re-calibrations of the detector.
Another method used for lowering the dependence of the sensibility from the accumulated dose, is that of intentionally introducing an impurity in the crystalline silicon by adding platinum, for example, at a concentration far greater than that of the defects being created; in this way, even prior to the radiation, the extension of diffusion of the minority carriers L is reduced and its value stabilized according to the dose of accumulated radiation.
Also these methods are not without drawbacks.
In particular, by adding the platinum as an impurity, it is difficult to uniform it over the whole silicon layer, so that it is not actually possible to produce a matrix of sensible elements with the same characteristics throughout the surface of the crystalline silicon layer.